Various methods and apparatus have been proposed for charging bulk material to a plurality of receiving stations, which are connected in series to a pneumatic conveyor conduit by way of associated separators or traps, from a delivery station connected to a material storage or supply container. The individual receiving stations, for feeding respective material-processing machines associated therewith are charged with material in succession by way of their separators, and the conveying action is interrupted by a blocking or shut-off signal which is triggered when the material deposited in the last separator reaches a predetermined level.
For example German Pat. specification No. 921,978 discloses an apparatus in which the conveyor conduit is a ring conduit through which excess bulk material, i.e. material which is not passed into the separators as they are already full, is returned to the supply container in a continuous circuit. A continuous circuit of this kind, with its additional power requirement, unnecessary wear of the conveyor conduit and its operating members, and any deterioration in the quality of the bulk material, can be avoided if the conveying action is interrupted by a blocking signal which is triggered in the last receiving station connected to the end of the conveyor conduit, when a maximum filling level is reached. For example, in the apparatus of German Offenlegungsschrift No. 1,506,965 the signal is triggered by the pressure rise which occurs when the level of material filling the last receiving station reaches the discharge aperture of the conveyor conduit. A fresh conveying operation is subsequently initiated by a timer.
A necessary condition for the operation of such an apparatus is that, before a new conveying phase begins, the last processing machine has consumed sufficient material that the discharge aperture of the conveyor conduit in the receiving station is again exposed, and the through-flow of conveying air to the exhaust filter at that station is ensured. In practice this means that, when for example a tool change or maintenance work is to be carried out on the last machine, the entire charging apparatus must be rendered inoperative, and this often results in an undesirable wast of time.
It would be possible to overcome this disadvantage by providing all the material-collecting containers forming the respective receiving stations with a filter which is of sufficient size to permit the entire amount of material-conveying air used to flow therethrough. However, this would result in very high costs and in a very substantial additional amount of space being required.
A further problem is posed by the charging of the individual receiving stations themselves; thus in known charging apparatus, the bulk material being conveyed to the stations for feeding processing machines is passed into the pneumatic conveyor conduit, at a feed station, from a storage or supply container, by means of compressed air pressure or by means of a metering or discharge apparatus. The pneumatic conveyor conduit is disposed above the material-collecting containers for receiving the material for the machines, and is connected to each container by way of branch connecting members. In the simplest case, each of these connecting members is formed by a container into which the mixture of bulk material and air is introduced radially. Each container is connected to a radial discharge line leading to the next following container. Separation of the conveyed bulk material occurs, if the container is sufficiently large, by sedimentation of the particles of material being conveyed, under the effect of gravity. It is apparent that when conveying fine dust materials, only a part of the material being conveyed will be separated out, and the remainder will be transported by the carrier gas to the next following machine.
However, as for example the plastics industry often uses mixtures of bulk materials, whose components differ from each other by different grain size and speed of fall, it can happen that primarily the coarse particles of material are separated out in the first separating container, and primarily fine material is separated out in the following containers. There will thus occur an undesirable break-down of the mixture into its components, during conveying.
In order to improve the degree of separation in the above-described gravitational separators, axial partitioning walls can be disposed transversely to the direction of the through-flow of air. Also known are separating devices in which the pneumatic conveyor conduit is passed downwardly into each collecting container and then turns through 180.degree. so as to extend upwardly out again; a branch conduit is connected to the lowest point of the curvature of the conveyor pipe conduit, and opens into the container. In this case separation of material occurs due to centrifugal force, the separated material flowing into the container through the branch conduit.
A further known separating device provides that the conveyor conduit, which is horizontal, opens into the respective collecting container, by way of a curved pipe portion which extends vertically downwardly to form an outlet. Upstream of the pipe curve is a branch connection which is taken out of the horizontal conduit at an angle of about 15.degree. to 90.degree., and which forms the continuation of the conveyor conduit. As long as the downwardly pointing outlet curved pipe portion is still empty, the greater part of the bulk material being conveyed will fly, due to mass inertia forces, through this pipe into the collecting container. Only when the outlet pipe is filled and there is a stream of material backed up to the main conveyor conduit, is the whole of the arriving bulk material conveyed to the next branch connection, where the separating operation is repeated.
These separating devices generally suffer from the disadvantage that they have a poor level of separation efficiency, and also can readily cause break-down of a bulk material mixture. Thus, it was found in a test run that with a bulk material which is used for making gramophone discs, in the best of the above-discussed known separating devices only about 70 to 80% of the incoming flow of material is separated off. The remaining amount of bulk material, which primarily comprises fine dust (20 to 30%), flows on to the next machine. In this flow of bulk material the fine-grain coloring agent component is substantially higher than in the material which was separated off, which results in undesirable differences in quality of the articles produced in each processing machine.